The present invention relates to an apparatus and to a process for measuring non-linear order three optical characteristics in an isotropic phase: susceptibility coefficient of order three and dispersion of indices.
The appearance of an order three light harmonic can be brought about by exciting a transparent body with the aid of a high intensity light beam, such as that produced by a laser. The energy supplied by the photons to the electrons of the body is in part restored in the form of order three harmonic light radiation with a highly non-linear intensity as a function of the exciting intensity and which is proportional to the square of a so-called order three susceptibility coefficient which is dependent on the material.
However, apparatuses aiming at measuring the characteristics of a body with respect to said order three harmonic emission are confronted with difficult precision problems, because the emission intensity also varies greatly as a function of the thickness traversed by the light beam with a period equal to that which is called the coherence length l.sub.c =.lambda..sub..omega. /6(n.sub.3.omega. -n.sub.107), in which .lambda..omega. designates the wavelength of the incident light of pulsation .omega. and in which n.sub..omega. and n.sub.3.omega. designate the refractive indices of the body at pulsation lights .omega. and 3.sub..omega.. This length is usually approximately 1 to 10 micrometers. The problem is even more difficult when working e.g. on liquid phases, which make the light pass through two walls of a tank containing the liquid to be analyzed. The harmonic three radiation collected is then the sum of the harmonic radiation emitted by the liquid and the products thereof by the walls, without it being possible to easily distinguish them.
Thus, an apparatus is known (Meredith, Buchalter, Hanzlik, Journal of Chemistry and Physics No. 78, p. 1533), according to which a parallel light beam passes through a tank of variable width filled by the body, whose characteristics are to be measured. This thickness of the tank walls is variable. The measurement consists of moving the light beam with respect to the tank, so as to traverse continuously variable thicknesses of walls and liquid. The light intensity of third harmonic varies and passes through maxima, whose height and periodicity make it possible to determine the dispersion of the liquid (n.sub.3.omega. -N.sub..omega.), as well as its non-linear susceptibility by comparison with a reference test on a body having known characteristics.
This process involves the creation of complicated interferences, which can only be dealt with by a computer. It is necessary to have a high accuracy on the angles characterizing the geometry of the apparatus and an excellent planeity of the surfaces of the tank walls.
An identical apparatus has been proposed (Thalhammer and Penzhofer, Applied Physics B32, p. 137), but whose walls have a constant thickness and are equal to an even multiple of the coherence length of the material of said walls with respect to the incident light. By forming a vacuum between these two walls, it is possible to show that they are together only able to emit a negligible third harmonic light intensity. However, this conclusion is again open to question if their gap is no longer occupied by the vacuum. The manfacture of walls with a perfect planeity and known thickness in a very accurate manner makes this process virtually unusable in practice, particularly as the coherence length is dependent on the light wavelength and consequently makes it possible to work with light having a given color. According to a third apparatus (Meredith, Buchalter and Hanzlik, Journal of Chemistry and Physics, No. 78, p. 1543), the liquid is disposed between two relatively well spaced walls, whose inner surfaces are parallel. However, the outer surface of the wall by which the light beam enters the apparatus is oblique with the result that the thickness of said wall is variable.
The beam is focused onto said wall and, as hereinbefore, the process consists of moving the beam along the apparatus. The third harmonic emitted has a variable intensity, because it is dependent on the thickness of the window. The harmonic emitted by the passage in the liquid remains constant, because it can be assumed that the liquid thickness is invariable. Even if the window by which the beam passes out is slightly oblique, this imprecision can be ignored, because the divergent beam is wide (low surface intensity) at this point and produces no non-linear intensity harmonic emission.
It is therefore very easy to determine the magnitude of the third harmonic emitted by the light, but the susceptibility coefficient can only be calculated on knowing the dispersion (n.sub.3.omega. -n.sub..omega.) of the liquid after performing another experiment. Thus, this simple and accurate apparatus is not complete. Moreover, it is less generally used than the previous apparatuses, because it uses a greater liquid thickness and which is consequently more opaque.
Moreover, these three apparatuses must be placed in a vacuum chamber, because the light beam is parallel or focused close to the surface of the apparatus, so that the external medium is at least locally traversed by an intense beam. Thus, an atmosphere would also emit a third harmonic, which would falsify the results no matter what the apparatus used.
Thus, in the state of the art these is no apparatus making it possible to analyse the third harmonic optical emission characteristics of a body in a simple, precise and economic manner.